The goal of this project is to study the etiology, heritability, diagnosis, and prevention of syncopal attacks and sudden death in a spontaneous animal model with similarities to sudden death in human adults and infants. The animal model consists of a line of purebred dogs with a tendency towards syncopal attacks and sudden death. Repeated episodes of atrial asystole have been documented using continuous monitoring of the electrocardiogram. The frequency of atrioventricular heart block was higher for the purebred line than for mixed breed dogs, and was associated with autonomic activity. The major objectives of the next year are to: 1) document the syncopal attacks using multichannel radiotelemetry to define the etiology of the spontaneous attacks and the susceptibility of individual dogs; 2) compare the baroreflex activity of the purebred line to mixed breed dogs; 3) compare the effects of rapid pacing on overdrive suppression of the purebred line compared to mixed breed dogs, and 4) evaluate the effect of pharmacologic agents on the frequency of attacks.